1. Field of Invention
The present invention relates to methods for making flowable, highly concentrated dry acifluorfen powders from an aqueous solution of the salt form of acifluorfen.
2. Background of the Prior Art
Herbicides may be applied to plants in a variety of methods including different formulations. Of these various methods, use of liquid and dry compositions are the most common. The particular formulation desired and resulting efficacy enhancement will greatly depend upon the species to be treated, environmental conditions, the geographical area and the climatology of the area at the time of treatment.
The herbicide, known trivially as acifluorfen, is widely used to control various weeds, such as morning glory, pigweed, ragweed, velvetleaf, foxtail, johnson grass, Florida beggarweed, cocklebur, and others. See, for example, U.S. Pat. Nos. 3,798,276 and 3,928,416.
Further patents claim enhanced herbicidal activity for acifluorfen and its agronomically acceptable salt forms with the addition of one water-soluble salt from an organic or inorganic acid (see, U.S. Pat. No. 4,508,559), and specifically by the addition of a citrate salt (see, U.S. Pat. No. 4,549,903).
Acifluorfen's chemical name is 5-(2-chloro-4-(trifluoro-methyl)phenoxy)-2-nitrobenzoic acid and is available from BASF Corporation under the trademarks BLAZER.RTM. or TACKLE.RTM.. These products are manufactured in the form of about a 43 to 48% active ingredient aqueous solution of the sodium acifluorfen with impurities (This product is sold by BASF Corporation under the trademark BLAZER.RTM. TECHNICAL).
The end user normally purchases acifluorfen either as an aqueous mixture of sodium acifluorfen, trisodium citrate, and a co-solvent butyl cellosolve (BLAZER.RTM.); or as sodium acifluorfen, a sodium lignin sulfonate--REAX 910 (from Westvaco Corporation), and co-solvent propylene glycol (TACKLE.RTM.).
Acifluorfen is preferentially sold in agronomically acceptable salt forms mentioned above because the salt form has a high water solubility which imparts convenience to the farmer who applies the product from this tractor tank. The sodium form, for example, has a water solubility of more than 119 g/L. The aqueous solution can be added directly to an application tank containing water, recirculated for a few minutes, and applied as a dilute aqueous liquid to the plant growth in the fields.
The acifluorfen in its acid form is not soluble in water to any appreciable extent, and presents dispersion problems using an application tank to apply the chemical universally in the field.
Until now, the high water solubility of the salt forms have given it an advantage over the water insoluble acid form for the end user. However, the high hygroscopicity of the salt form makes it difficult to dry the salt form. The water is difficult to remove during drying.
Attempts to remove the water from acifluorfen formulations by spray drying have resulted in a sticky, non-flowable mass, most of which stuck to the walls, ducts, and cyclone walls of the spray dryer, rendering the process ineffective.
Similarly, attempts to dry acifluorfen formulations, such as solutions of REAX 910 with BLAZER.RTM., by use of a double drum dryer resulted in the formation of a sticky material that could not be scraped off the drums.
Furthermore, drying generally is also hindered by foaming which can result in certain dryers being ineffective. In batch drying for example, foaming can greatly hinder the rate of drying, or even eliminate the possibility of drying such a compound sufficiently to form a powder. All three commercial formulations of acifluorfen, (BLAZER.RTM. Technical, BLAZER.RTM., and TACKLE.RTM.) could not be dried in several batch driers. Attempts to dry these products in either a rotary dryer or a mechanical fluid bed dryer with chopping blades (for example, the Littleford FKM dryer) resulted in violent foaming, which rendered the process ineffective.
Generally, solid forms of herbicides offer a number of key advantages, including convenience, increased stability and shelf life, as well as reduced packaging, storage and shipping costs. Additionally, there is the possibility of future government regulation requiring solid forms of agricultural products in order to reduce handling of contaminated packaging of these products during field application and during disposal. These dry flowable hygroscopic herbicidal compounds would be safer for the farmer to use and dispose of, and also result in a smaller volume of hazardous waste being produced. If the dry herbicide can be filled into dissolvable bags which are added to the tractor tank, then exposure during loading and the hazardous waste generated can be virtually eliminated.
There is a need for a dry, flowable, highly concentrated powder form of hygroscopic acifluorfen suitable for filling water dissolvable bags. The ideal dry powder would not only include the active ingredient, but also a sequestering agent such as a citrate salt.